This invention relates to an echo controller, as called herein, for obviating an undesirable echo signal in a long-distance telephone network.
A long-distance telephone network generally comprises a plurality of two-wire and four-wire circuits or lines and a hybrid coil or circuit at each junction of the two-wire and four-wire lines. Each two-wire line serves as both of a signal receiving and sending path. Each four-wire line comprises an incoming or receiving path and an outgoing or sending path. Due to inevitable mismatch of impedance, the hybrid circuit delivers a fraction of a telephone or voice signal sent from the four-wire line to the two-wire line, namely, from a remote subscriber to a local subscriber, as an undesirable echo signal back to the remote subscriber. The hybrid circuit thus unavoidably serves as an echo path. It is conventional in order to obviate the echo signal to use either an echo suppressor or an echo canceller in the vicinity of each junction. A first of three terminals of a conventional echo canceller or suppressor for receiving a signal from the receiving path of the four-wire line, a second thereof for receiving a signal from the sending path thereof, and a third thereof for sending a signal to the sending path are generally known as a receiver-in (or received), a send-in (or unprocessed), and a send-out (or transmitting) terminal, respectively. The signals at these terminals are called in terms of the corresponding terminals.
A conventional echo suppressor is responsive to a receive-in (i.e. received) and a send-in (i.e. unprocessed) signal and produces an echo suppressed signal as a send-out (i.e. transmitted) signal. An echo suppressor is featured by its simple structure but is disadvantageous in that it irresistively introduces defects or impediments into conversation, such as omission of beginning of an interrupting voice signal sent from the local subscriber to a remote subscriber during presence of an existing voice signal sent from the latter to the former.
A sophisticated echo canceller comprises an echo simulator responsive to a receive-in signal for producing a simulated or synthesized echo signal or echo simulation signal and a subtractor for subtracting the echo simulation signal from a send-in signal to produce an echo cancelled signal as a send-out signal. The send-in signal is referred to as an "unprocessed" signal since it has yet to undergo either echo cancellation or echo suppression as will be more fully described. The echo simulator retains a set of parameters determined in compliance with approximated or estimated characteristics of the echo path. The echo cancelled signal consists of a residual echo signal when there is no double talk in the lines, namely, when the local subscriber does not speak to a remote subscriber while the latter is already speaking to the former. The residual echo signal should ideally be of a zero level but provides in practice a finite cancellation error. Inasmuch as the characteristics of an echo path may fluctuate from time to time, it is preferred that an echo canceller further comprises self-adaptive means responsive to the residual echo signal for adjusting or modifying the echo simulator so as to produce a least possible residual echo signal. Self-adaptively adjusted, the echo simulator changes the echo simulation signal when the current characteristics of the echo path becomes different from the previously estimated characteristics of the echo path. An echo canceller is theoretically preferred to an echo suppressor because of absence of the above-mentioned impediments but loses its function under certain circumstances, such as when a large noise appears in the lines, when the echo path characteristics are nonlinear, and/or when a large variation occurs in the echo path characteristics.
In U.S. Pat. No. 3,754,105 issued to Poschenrieder et al., a circuit arrangement is disclosed wherein an echo suppressor is used in cascade after a self-adaptive echo canceller in the sending path of a four-wire line with a view to obviating the loss of function of the latter. The echo suppressor receives as its send-in signal the send-out signal of the echo canceller and is rendered operative only when the echo simulator of the echo canceller is self-adaptively adjusted beyond a predetermined amount due to the line noise, nonlinearity of the echo path characteristics, and/or fluctuation in the echo path characteristics. As will readily be understood, the echo simulator is disturbed or adversely adjusted to a considerable extent at the beginning of double talk even if the echo path is of excellent characteristics, namely, little subject to noise, of considerably linear characteristics, and of little fluctuating characteristics. As a result, the circuit arrangement still develops the inherent defects of an echo suppressor at the beginning of double talk. The echo simulator is considerably adjusted also during presence of a large fluctuation in the echo path characteristics. When double talk occurs in this circumstance, the echo simulator is unduely disturbed. This causes a disturbance to the echo canceller output signal and, in some cases, produces an amplified echo signal therein. Responsive to such a send-in signal, the echo suppressor is liable to send a contaminated send-out signal to the sending path.